Currently scheduling a refresh of Excel files with PowerPivot models is not yet supported. If you are having problems viewing the excel document, ensure you have Office Online server configured correctly with the report server.
We established an Analysis Services with PowerPivot mode, but problem is still continuing. We made all the instructions on Microsoft's document page, but it still does not work. Probably we are not able to tell the PBIRS that it needs to use this Analysis Services with PowerPivot mode.
Recently, we tested Power BI Report Server + Office Onine Server. Excel connected to SSAS or Excel with Power Pivot all work fine. We encountered your error message as well. Check SQL server version, if SQL2017, upgrated to CU3 or higher. The error was cause by missing peroformace counter( You can find error message in ULS log).